[REBOL] Re: compose: what do not understand?
From: joel::neely::fedex::com at: 3-Oct-2001 10:36
Hi, Romano,
Romano Paolo Tenca wrote:
> Someone can explain to me where these two command differs?
> Aren't the same thing?
>
> >> x: []
> == []
> >> compose [(:x)]
> == []
> >> reduce [:x]
> == [[]]
>
Pardon me if I answer with too much detail...
The essential difference in REDUCE and COMPOSE is that you get
to decide what gets evaluated:
>> fee: "eenie" == "eenie"
>> fie: "meenie" == "meenie"
>> foe: "mynie" == "mynie"
>> fum: "moe" == "moe"
>> reduce [fee fie foe fum]
== ["eenie" "meenie" "mynie" "moe"]
>> compose [fee (fie) foe (head reverse copy fum)]
== [fee "meenie" foe "eom"]
In addition, COMPOSE flattens its content, in the following way:
>> p: [1 2 3] == [1 2 3]
>> q: [4 5 6] == [4 5 6]
>> r: [7 8 9] == [7 8 9]
>> reduce [p q r] == [[1 2 3] [4 5 6] [7 8 9]]
>> compose [(p) (q) (r)] == [1 2 3 4 5 6 7 8 9]
and also eliminates empty blocks, as follows:
>> s: [] == []
>> reduce [p q s r] == [[1 2 3] [4 5 6] [] [7 8 9]]
>> compose [(p) (q) (s) (r)] == [1 2 3 4 5 6 7 8 9]
HTH!
-jn-
--
This sentence contradicts itself -- no actually it doesn't.
-- Doug Hofstadter
joel<dot>neely<at>fedex<dot>com
